Modern semiconductor technology depends upon methods for growing a multiplicity of high quality semiconductor layers on, for example, a substrate or other epitaxial layers. The layers should have few residual, i.e., undesired, impurities and carrier traps as well as few structural imperfections. Accordingly, several methods have been developed to grow such epitaxial layers. The layers may comprise elemental semiconductors, such as silicon or germanium, or compound semiconductors such as Group III-V or Group II-VI binary, ternary, or quaternary semiconductors.
For the growth of Group III-V compound semiconductor materials, the first method developed and brought to a high degree of perfection was liquid phase epitaxy (LPE). While this method is now well developed and perfectly adequate for the fabrication of many types of epitaxial layers and devices comprising at least one of such layers, it is not without its limitations. For example, LPE requires that a substrate be moved from one melt to another, etc., with epitaxial growth typically occurring in each melt. The melts are typically compositionally varying, and therefore, precautions generally have to be made to prevent undesired transport of melt constituents from one melt to another melt. While this limitation may be overcome by careful apparatus design and operation, other limitations appear more fundamental and difficult to overcome. For example, fabrication of ultra-thin layers, for example, less than 200 Angstroms thick, is usually difficult, if not impossible, with this technique. Even very thin, less than 1000 Angstroms thick, smooth layers have been difficult to grow reproducibly. Additionally, interfaces between layers having very abrupt, i.e., step function, compositional or doping variations are also difficult to fabricate.
As a result of these and other limitations, as well as for other reasons, additional techniques have been developed for the growth of Group III-V compound semiconductor materials. The most promising of these additional techniques at present appears to be molecular beam epitaxy (MBE). This method is described in detail in U.S. Pat. No. 3,615,931 issued to John R. Arthur, Jr. on Oct. 26, 1971. In MBE, effusion ovens containing the desired Group III and Group V materials are heated to a temperature sufficient to volatilize the materials and the resulting thermally evaporated beams are directed toward the substrate upon which epitaxial growth is desired. The substrate is maintained at a temperature that is high enough for surface diffusion and epitaxial growth to occur but is low enough so that the materials have a reasonable probability of sticking to the surface. The beams have a cosine square distribution in intensity and when the basic method is supplemented with other techniques, such as substrate rotation, permit growth of extremely compositionally uniform epitaxial layers over large diameter substrates. Furthermore, molecular beam epitaxy permits fabrication of, for example, extremely thin, in fact, even monolayer, epitaxial layers as well as interfaces having very abrupt compositional and doping variations.
The beams used by molecular beam epitaxy are generally electrically neutral. However, nonthermal charged particle, i.e., ion, beams have been used in at least several semiconductor epitaxial growth techniques. The use of nonthermal ions offers, at least theoretically, the possibility of lower substrate temperatures during growth because the kinetic energy of the deposited particles enhances surface diffusion. The preparation of InSb thin films by an ion beam epitaxy technique is described in Journal of the Vacuum Society of Japan, 20, pp. 241-246, July 1977. The technique described ionized both In and Sb particles. The ions were accelerated by a high constant voltage, 1000 volts or greater, applied to the substrate. Uniform area growth, within unspecified tolerances, was apparently demonstrated. However, for purposes of compositional control, flash evaporation was employed, i.e., the composition of the layer deposited was controlled by flash evaporation to completion of preweighed In and Sb charges. The preweighing was required because both the In and Sb particles were charged and thus had high sticking coefficients on the substrate surface. Consequently, the deposited layer might not have perfect stoichiometry as either In or Sb might be incorporated into the layer in excess.
Epitaxial growth of silicon on either germanium, Ge(100), or silicon, Si(100) or Si(111), substrates using ion beam epitaxy is described in Applied Physics Letters, 41, pp. 167-169, July 15, 1982. The method described used silicon ions having energies between 50 and 100 eV and obtained epitaxial silicon growth at substrate temperatures between 300 and 900 degrees K. The silicon beam was formed by thermal evaporation of silicon from an effusion oven, and a discharge voltage of 60 volts accelerated the particles from the plasma. Growth of thin and high-quality silicon layers was reported. However, this method permits the growth only of elemental semiconductors.
As is evident from the preceding discussion, molecular beam epitaxy is not generally a selective area growth technique because the molecular beams impinge upon the entire crystalline substrate, i.e., the beams cover the entire substrate surface. Techniques have been developed which modify the basic molecular beam epitaxy technique described by Arthur to permit selective area growth. However, these techniques require steps, for example, suitable substrate preparation, which necessitate additional processing complexity. See, for example, U.S. Pat. No. 3,982,092 issued on Dec. 23, 1975 to William Charles Ballamy and Alfred Yi Cho. The method described by Ballamy and Cho prepares planar isolated devices by forming an amorphous insulating layer on a Group III-V substrate and removing selected portions of the layer to expose the underlying crystalline material thus yielding a patterned substrate. These steps are performed outside the MBE growth chamber. The now patterned substrate is transported into the MBE growth chamber for deposition of material by MBE. Single crystal material grows only on the exposed portions of the substrate. Other selective area growth techniques, for example, mechanical shadow masking, are known.
Ion beams have been used for other purposes in epitaxial growth processes. For example, Applied Physics Letters, 40, pp. 686-688, Apr. 15, 1982, describes the use of a high energy ion beam in silicon fabrication. Amorphous silicon is initially deposited. A high energy, approximately 2.5 MeV, arsenic ion beam is then directed to the amorphous silicon layer. The resulting heating causes the recrystallization of the amorphous silicon with the impurity arsenic atoms from the ion beam located on substitutional sites within the crystal lattice.